Section style should be framed as a special section

Original issue: https://github.com/asciidoctor/asciidoc-docs/issues/5

When a section title has a style (e.g., preface, appendix, glossary, etc.), it automatically makes that section a special section. The documentation should introduce the term "special section" and explain this correlation.

Also, do we need separate pages for Dedication and Colophon? Except for a handful of exceptions, special sections are just a passthrough designation (What does a passthrough designation actually mean?). For any special section style that AsciiDoc doesn't apply special meaning, they should be grouped on the Special Sections page and just listed. These include: colophon, dedication, acknowledgements. We may also point out that technically the list is open ended. It's up to the discretion of the converter to handle any additional names.